This invention relates to light projectors and more particularly to a lamphousing including improved apparatus for magnetically stabilizing the arc in a horizontally operated high-pressure short-arc xenon lamp.
In recent years, carbon arc light sources for light projectors such as are used with motion picture projectors and for theatrical stage lighting have often been replaced with gaseous discharge lamps, such as the high-pressure short-arc xenon lamp, mounted in a lamphousing. Lamps of this type are not only more efficient than carbon arc lamps, but they can also produce a whiter light. In early projectors using short-arc xenon lamps, it was necessary to orient the lamp with the electrodes vertically aligned and the larger anodes spaced above the smaller cathode. This lamp orientation is inefficient since the collecting mirror cannot effectively collect the light being generated 360.degree. around the arc light source. It is preferable to operate the lamp in a horizontal orientation since considerably more light can be collected and distributed properly across the film aperture plate. However, if the lamp is tipped from a vertical electrode orientation, convection currents within the gas in the lamp envelope cause the arc to rise and also cause the projected light to flicker or move about. This in turn causes the electrodes to wear at an uneven rate. More importantly, a hot spot occurs on the portion of the lamp envelope above the arc or plasma flame surrounding the arc which greatly increased the rate at which the lamp envelope devitrifies. If the plasma flame actually touches the envelope, the envelope softens or melts to a point where the high internal gas pressures cause the lamp to shatter.
Three methods are known for stabilizing the arc or plasma flame in a xenon lamp to permit operating the lamp with the electrodes in a horizontal orientation. By operating the lamp at a very intense current level, self-magnetism causes the plasma to be "stiff" and stay substantially centered between the electrodes. However, the very high currents are inefficient and decrease the lamp life. When a lamp manufacturer specifies that a lamp may be operated in a horizontal orientation, the lamp has a high current rating. Typically, the lamp must be operated at at least 80% of its rated current when horizontal. There is a high risk of destruction of the bulb if the lamp current is substantially decreased below the rated current.
Another method for stabilizing the arc is disclosed in U.S. Pat. No. 2,757,277 which issued on July 31, 1956. Here, the lamp is mounted on a support which is continuously rotated to prevent overheating the top of the lamp envelope. However, the lamp support is complicated and would not be practical for many installations.
U.S. Pat. No. 2,757,277 also indicates that the lamp may be stabilized with a magnetic field-producing device. However, it is pointed out that the magnetic field-producing devices needed to be located in areas where they obstructed useful light produced by the lamp. Such an arrangement is shown in U.S. Pat. No. 3,624,386, which issued on Nov. 30, 1971. This patent discloses a lamphousing in which the horizontal cathode of an arc lamp passes through an opening in the center of a reflector in a conventional manner. Either a permanent magnet or an electromagnet is mounted below the lamp on an adjustable bracket for arc stabilization. However, the magnet is also located below the arc plasma in the region between the reflector and a light outlet port on the lamphousing. Therefore, a portion of the useful reflected light is lost. Generally, lamphousings having magnetic arc stabilization have also used expensive glass reflectors since metallic reflectors would have interfered with the magnetic field and therefore interfered with the arc stabilization. Another problem encountered in lamphousings of this type has been with initial magnet adjustment. Since the magnet is located within the reflector, an operator is exposed to the hot lamp while initially adjusting or locating the magnet and he is also exposed to injury if the lamp should explode.